Medical devices configured for minimally invasive surgery are rapidly becoming the tools of choice for many surgical procedures. Not only do these devices provide an alternative to more invasive surgical tools and procedures, but they have also fostered the development of entirely new procedures.
Devices including highly flexible catheters, as well as rigid and semi-flexible probes have received increased attention in recent years and continue to be refined for cardiovascular, pulmonary, urogenital, and other applications. Devices for each of these applications present different technology and material challenges. Such catheters, for example, can require fluid-tight passages or channels for circulating a cooling fluid (liquid gas) to cool an electro-surgical structure, such as radio frequency ablation electrode, to prevent overheating of the electrode or of surrounding tissue. Similarly, a cooling or cryogenic fluid can be used to reduce the temperature of a structure, such as an ablation surface, to a therapeutic temperature.
Such devices are also useful for procedures involving stenosed arteries and/or blood vessels. As used herein, stenosis means constriction or narrowing. A coronary artery that is constricted or narrowed is thereby referred to as stenosed. These arteries or vessels may be clogged by the buildup over time of fat, cholesterol and other substances.
One procedure for widening coronary arteries is a type of angioplasty, typically a balloon angioplasty. However, many patients undergoing typical angioplasty procedures have renewed narrowing or restenosis of the widened segment within months of the procedure. Restenosed arteries then have to be rewidened.
Restenosis can also occur after a coronary artery bypass graft operation. This type of heart surgery is done to reroute, or "bypass," blood around clogged arteries and improve the supply of blood and oxygen to the heart. In this case, the stenosis may occur in the transplanted blood vessel segments. Like other stenosed arteries, they may have to undergo procedures to reopen them.
Minimally invasive catheters, especially cryogenic based minimally invasive is catheters are well adapted for electrophysiology and restenosis applications. However, the small diameters at which these cryogenic catheters have to be made may result in possible clogging or reduced fluid flow within the device because of the reduced cross sectional areas. The inclusion of guide wires, thermocouple wires, multiple lumens and other features within the cryogenic catheter all also reduce the cross sectional area inside catheter which impedes the space within the catheter that is available for return flow for the fluid within the catheter.
Furthermore, these reduced flows within the catheter makes it difficult to freeze given lengths of the catheter. These limitations may be overcome by forcing cryogenic fluid at relatively high pressures through the catheters. However, such techniques can result in a high pressure differential existing within the catheter. This pressure differential may cause some safety problems since in the case of a component failure, e.g. a leakage, it is desirable to have the tip pressure as low as possible within the catheter. Performance problems may also result since the temperature of the expanding refrigerant is proportional to the ambient pressure at which it expands.
Accordingly, it would be desirable to provide a medical device such as a cryogenic catheter with a slidable injection tube which improves return fluid flow within the catheter and allows given lengths to be frozen without the safety and performance problems of prior art catheters.